Multiple factors determine whether the net synthesis of proteins and mass of skeletal muscle fibers increase, decrease of remain constant. The primary objective of this proposal is to determine the contribution of neural (activity-independent neurotrophic) and mechanical (tension) factors as independent and as interactive factors in maintaining the functional, structural and metabolic integrity of skeletal muscles that have been subjected to 60 days of complete inactivity. Whole muscle and single fibers in the adult rat soleus, a slow extensor known to be affected dramatically by decreased used, will be studied. A unique aspect of the proposal is that a true baseline of inactivity will be produced by spinal cord isolation (SI), i.e. spinal cord transection at a mid-thoracic and high sacral level and bilateral dorsal rhizotomy between the two transection sites. In one group of rats, intramuscular electrodes will be implanted in several hindlimb muscles to monitor EMG activity throughout the experimental period. To define the level of control attributable to activity-independent neurotrophic factors, a section of the lateral gastrocnemius-soleus nerve will be removed in some groups of SI rats. In some SI rats, stimulating electrodes will be implanted near the lateral gastrocnemius-soleus nerve (nerve-intact group) to isometrically stimulate (40 Hz, 330 ms train, once/second) the soleus twice daily for 30 minute/session. Tendon force transducers will be implanted acutely on the distal soleus tendon to quantify the forces imposed on the muscle during the stimulation protocol. After 60 days, contractile, morphological, biochemical and/or histochemical properties of the whole muscle and/or single fibers will be determined. The regulation of type I myosin heavy chain gene expression at the transcriptional-pretranslational level associated with SI and/or denervation, with and without mechanical stimulation, will be determined 28 days after SI. Since fiber size can be modulated concomitantly with the number of myonuclei, the role of apoptosis, i.e. programmed cell death, and satellite cell activity in the atrophic response and the expected amelioration of atrophy with resistance exercise will be determined at two time points, i.e. 8 and 17 days after SI. In these studies we will identify the relative importance of the control of muscle mass and phenotype from a) within the muscle fiber (intrinsic, no neurotrophic); b) the nerve supply without activity (neurotrophic); and c) the nerve supply with controlled activity and force patterns. Clinically, the results will demonstrate the efficacy of short, daily periods of programmed mechanical stimulation as a preventative and/or rehabilitative tool for patients with spinal and neuromuscular maladies.